This application is claiming priority to Japanese Application No. 2000-245539 filed Aug. 8, 2000, herein incorporated by reference in its entirety. The present invention relates to a coating device for forming a coating layer on a base web material, particularly to a coating device including a back-up roll carrying on an outer surface thereof a moving base web material and rotating therewith, and a coating head spaced from the back-up roll for continuously coating a surface of the moving base web material with a coating material.
There has been known, as a method for coating a surface of a base web material having a continuous length and a predetermined width such as resin film, rein sheet or paper with a coating material, a so-called die coating method wherein a nozzle section of an extrusion die is used as a coating head for continuously extruding a fluid coating material having a predetermined viscosity therefrom and forming a coating layer with predetermined thickness on the moving base material. The die coating method is advantageous in comparison with other known roll coating methods or knife coating methods when it is desired to obtain a thin coating layer with a uniform thickness. Die coating methods permit the formation of a thin coating layer in a precise and stable manner. In die coating applications, the thickness of the coating layer is directly adjustable by the adjustment of an extrusion rate of the coating material. In addition, the die coating methods prevent the premature drying of the coating material on the coating applicator thereby preventing associated problems such as streaking or uneven color in the resulting web.
One form of die coating method incorporates the use of a back-up roll type coating device. Back-up roll type coating devices generally involve a moving base web carried on a back-up roll and coated with a coating material supplied from an end surface; i.e. a coating face; of a nozzle of a coating head spaced at a predetermined distance from the surface of the base web. In the past, back-up roll type coating devices extrude a measured amount of coating material from the coating head through a small gap defined between the coating face of the coating head and the surface of the base web carried on the back-up roll. As a result, a coating layer of a predetermined thickness is continuously formed on the surface of the moving base web. An extremely sharp edge of a high linearity is formed at a downstream end of the coating face of the coating head as seen in the moving direction of the base web for the purpose of smoothing a surface of the coating layer. In this coating device, a thickness of the coating layer formed on the base web is controllable by adjusting both of an extrusion rate of the coating material and a distance between the coating head and the back-up roll.
Generally in the above-mentioned coating device, the coating head is provided with a coating material supply passage communicating with the coating face, through which the coating material is continuously supplied to the coating face in an extruding manner. The coating material supply passage opens at the coating face in a form extending in the transverse direction generally vertical to the moving direction of the base web, whereby a coating layer of a strip shape with a predetermined transverse dimension is formed on the surface of the base web. In this case, the coating material is supplied to the coating face under the interior pressure in the coating material supply passage, and applied to the surface of the base material while being pressed in the small gap (or a coating gap) between the coating face downstream from the coating material supply passage as seen in the moving direction of the base web and the surface of the base web carried on the back-up roll.
Another coating device of a back-up roll type is conventionally recognized as a lip coater or die coater. With a lip coater the coating material supply passage is sufficiently extended to cause the coating material supply passage to dwell therein so that the coating material is pressed substantially solely in the coating gap.
As described above, according to the back-up roll type coating device, a coating layer of a desired thickness is formed on the surface of the base web by delivering the coating material supplied to the coating face of the coating head onto the moving base material at a constant extrusion rate while pressing the coating material within the coating gap having a predetermined dimension. Accordingly, if particles or air bubbles of a size exceeding the dimension of the coating gap (generally several hundreds xcexcm) are contained in the supplied coating material, the particle or air bubbles might block the coating gap to result in a linear pattern (or an undesirable streak) continuously extending in the moving direction of the web. If the coating head has the above-mentioned coating material supply passage, the blocking phenomenon caused by particles or air bubbles contained in the coating material may occur also in the coating material supply passage.
With conventional coating devices, when the streak occurs in the coating layer during the coating operation, the coating operation is suspended and a cleaning operation is carried out to remove the causative material for the blockade from the coating face or the coating material supply passage. For this purpose, the prior art coating device is provided with a shifting mechanism for shifting the coating head close to and away from the back-up roll so that a sufficient distance is obtained between coating face of the coating head and the base material carried on the back-up roll for carrying out the cleaning operation.
In the above-mentioned conventional back-up roll type coating device, the cleaning operation carried out when the undesirable streak occurs in the coating layer during the coating operation is, for example, an operation for manually wiping off the coating material from the coating face or scraping out the coating material from the coating material supply passage while suspending the supply of the coating material to the coating head after shifting the coating head to a position sufficiently remote from the back-up roll by driving the shifting mechanism. Such a cleaning operation itself is troublesome and requires much time and labor. In addition, the coating material may leak out on the coating face of the coating head to contaminate the same before the coating head resumes the operative position closer to the back-up roll after the completion of the cleaning. In such a case, there is a risk in that the coating material thus leaking is partially dried up to cause a new undesirable streak when the coating operation starts again.
It is actually difficult to completely remove such a leaking coating material by a manual operation. Because of the cleaning operation, not only a total time necessary for the coating operation is uselessly extended but also the coating material is wastefully consumed, which causes the production cost of a product with a coating layer to rise. Further, when the cleaning operation is carried out without stopping the movement of the base material to shorten the time necessary for the cleaning operation as much as possible, a portion of the base material having no coating layer must be treated as a waste. This further increases the production cost of the product with a coating layer due to the material loss.
An object of the present invention is to provide a high-performance coating device for carrying out the die coating method, capable of solving the above-mentioned problems of the prior art coating device of a back-up roll type, quickly and assuredly eliminating undesirable streaks in the coating layer if generated during the coating operation, and producing a product with a coating layer having a good quality (thickness uniformity, surface smoothness, presence of fault or others) while reducing the loss of coating material or base material to prevent the production cost from rising.
To achieve the above object, according to the invention, a coating device is provided, comprising a back-up roll carrying on an outer surface thereof a moving base web material and rotating therewith, a coating head spaced from the back-up roll and including a coating face opposed to the outer surface of the back-up roll, a coating material being continuously supplied onto the coating face, and a shifting mechanism for shifting the coating head relative to the back-up roll, wherein the coating material is measured between the base web material carried on the outer surface of the back-up roll and the coating face of the coating head so that a coating layer with a predetermined thickness is continuously formed on a surface of the moving base web material, characterized in that the shifting mechanism can reciprocally shift the coating head to temporarily increase a distance between the outer surface of the back-up roll and the coating face of the coating head during a coating operation.
According to an implementation of the invention, in the coating device the shifting mechanism comprises a drive section for shifting the coating head between a first position where the coating head is close to the back-up roll to perform the coating operation and a second position where the coating head is remote from the back-up roll to suspend the coating operation, and a stop section for selectively hindering a movement of the coating head at a third position close to the first position during the movement of the coating head from the first position toward the second position caused by the drive section.
According to an implementation of the invention, in the coating device the drive section of the shifting mechanism acts to return the coating head from the third position to the first position just after the movement of the coating head is hindered by the stop section.
According to an implementation of the invention, in the coating device the stop section of the shifting mechanism includes a stopper member capable of selectively shifting between an operative position where the movement of the coating head is hindered and an inoperative position where the movement of the coating head is allowed.
According to an implementation of the invention, the stop section of the shifting mechanism can adjust a distance between the first and third positions of the coating head.
According to an implementation of the invention, the drive section of the shifting mechanism linearly shifts the coating head between the first and second positions.
According to an implementation of the invention, in the coating device the drive section of the shifting mechanism includes a cylinder unit and a linear guide.
According to an implementation of the invention, the shifting mechanism further comprises a position adjust section for optimizing a distance between the outer surface of the back-up roll and the coating face of the coating head when a coating operation is performed.
According to an implementation of the invention, in the coating device the position adjust section of the shifting mechanism includes a feed screw unit.
According to an implementation of the invention, the coating head further includes a coating material supply passage communicating with the coating face, the coating material being continuously supplied onto the coating face throughout the coating material supply passage in an extruding manner.